Optical logic arrangement

ABSTRACT

An arrangement for performing an optical logic operation. The optical logic arrangement comprises a plurality (100) of reflection holograms (101-109) positioned in a two-dimensional array for optically interconnecting a similar plurality (120) of optically nonlinear optical devices (121-129) also positioned in a two-dimensional array. Each device is responsive to control light beams received on either side of the device array for emitting an output light beam that is a nonlinear gain function of at least one control light beam applied to the device. Accordingly, each device can regenerate light beams and perform an optical logic function. In response to an output light beam from a specified optical logic element, the associated reflection hologram originates an individual control light beam to one or more designated optical logic elements in the logic array. Consequently, the optical logic elements may be optically interconnected to perform any combination of sequential and combinational logic operations including those, for example, of an optical digital processor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to the applications of H. S. Hinton et al.,entitled "Optical Interconnection Arrangement", Ser. No. 683,716; H. S.Hinton, et al., entitled "Optical Logic Arrangement with SelfElectro-Optic Effect Devices", Ser. No. 683,711; and H. S. Hinton etal., entitled "Method and Apparatus for Making a device for opticallyinterconnecting optical devices", Ser. No. 683,712, all filedconcurrently with this application on Dec. 19, 1984.

TECHNICAL FIELD

This invention relates generally to logic arrangements and, moreparticularly, to an optical logic arrangement for performing an opticallogic operation.

BACKGROUND OF THE INVENTION

A vast number of prior art logic circuits employ optical devices toperform optical logic functions and operations. However, many of thesecircuits involve converting logic level signals between the opticaldomain and another domain such as the electrical domain. This opticalconversion process limits the bandwidth of the optical signals, requiresadditional processing time, and commonly requires additional circuitry.Generally, the output signal of a nonlinear optical device is anonlinear gain function of an input signal applied to the device whereeither the input or the output signal is in the optical domain. In themore specific case of an optically nonlinear optical device, the inputand the output signals are both in the optical domain. Consequently,optically nonlinear optical devices can regenerate optical signals andperform optical logic functions such as the optical logic NOR, OR, NAND,and the like. However, optically nonlinear optical devices that employan optical conversion process still have the aforementioned problems.

With the use of parallel processing techniques, it is often desirable toconnect in a parallel manner the optical output of each optical logicelement in one array to the optical input of each optical logic elementin another array. As a result, the number of individual physicalconnections using, for example, optical fibers between the two arrayscan be enormous with the total equaling the mathematical product of thenumber of elements in one array times the number of in the other array.Depending on the physical size of the elements as well as theinterconnections, space considerations can rapidly become a factorlimiting the number of connections between two arrays. This is just onereason why optional parallel processing techniques have had such limitedacceptance and use.

Another prior art approach for interconnecting optical logic elementsuses a computer-generated transmission hologram. Generally, a hologramconsists of any material for storing the optical wavefront from anobject that is encoded in an optical fringe pattern for subsequentrecreation of the wavefront. One familiar example of a hologram forcreating artistic visual effects is a photographic plate that has beenexposed to coherent light from a three-dimensional object and areference beam interfering in the plate. After the photographic plate isdeveloped, the reference beam is passed through the developedphotographic plate to recreate a three-dimensional image of the object.

One example of an optical sequential logic system utilizingcomputer-generated transmission holograms for optically interconnectingthe optical logic elements of the system is described by A. A. Sawchuket al. in Technical Report No. 1100 entitled "Nonlinear Real-TimeOptical Signal Processing", University of Southern California ImageProcessing Institute, Los Angeles, Calif., 1983. The optical logicsystem includes an array of computer-generated Fourier transmissionholograms for optically interconnecting a similar array of liquidcrystal light valves. The light valves are optically nonlinear opticaldevice and are operated to regenerate optical signals and to perform anoptical logic NOR function. However, one disadvantage of the liquidcrystal light valve is that the optical input control signals arereceived on one surface of the device and that the optical outputsignals are emitted from another surface usually on the other side ofthe device. Thus, the transmission holograms and a complicatedarrangement of precisely positioned lenses and mirrors must direct theoptical output signals from the rear surface of the light valve array360 degrees onto the front surface of the light valve array. The longdistance that optical output signals must travel from the rear surfaceof a light valve before being reflected as an input control signal ontothe front surface of at least one other light valve, severely limits theoperating speed of any optical system using this transmission holograminterconnection arrangement. Another problem with this interconnectionarrangement is the mechanical precision required in aligning thetransmission holograms and the light valves with the mirrors and thelenses. A slight vibration can misalign the entire system.

Another disadvantage of the liquid crystal light valve is its relativelyslow switching speed.

Another problem with Fourier transmission holograms is the significantpower loss of an optical signal as it passes through the hologram. Eachoptical signal passing through a Fourier transmission hologram forms twoimages of which only one is used to interconnect the light valves andhas at most half the optical power of the incident signal. Furthermore,transmission holograms operate only with coherent light which may resultin optical interference at the input of an optical logic between theseveral input signals.

SUMMARY OF THE INVENTION

The foregoing problems of transmission hologram optical logicarrangements are solved and a technical advance is achieved by anoptical logic arrangement comprising optically nonlinear optical devicesoptically interconnected by a reflection hologram. Each device isresponsive to a control light beam for emitting an output light beam. Inresponse to an interconnecting output light beam from one device, thereflection hologram originates an interconnecting control light beam toanother device. The reflection hologram can also be made to originateindividual interconnecting control light beams to two or more opticallynonlinear optical devices in response to an interconnecting output lightbeam from one device.

Advantageously, with each optically nonlinear optical device operated toperform an optical logic function, a plurality of these devices may beoptically interconnected to form an optical logic circuit or to performan optical logic operation.

In one illustrative embodiment of this invention, the reflectionhologram originates an interconnecting control light beam to adesignated optically nonlinear optical device by reflecting apredetermined amount of the interconnecting output light beam from aspecified device to the designated device.

In another illustrative embodiment of this invention, a plurality ofreflection holograms positioned in a two-dimensional array opticallyinterconnects a plurality of optically nonlinear optical devices alsopositioned in a two-dimensional array to form a two-by-two opticalcrossbar switch. Each optically nonlinear optical device such as anonlinear Fabry-Perot Interferometer is operated to function as anoptical logic NOR gate. Furthermore, an optically nonlinearinterferometer can switch advantageously at a much faster rate than aliquid crystal light valve. According to one feature of this invention,each optically nonlinear optical device is responsive to light beamsincident on either surface of the two-dimensional logic element arrayfor emitting an interconnecting output light beam to a specifiedreflection hologram.

Advantageously, without the use of mirrors or lenses, each reflectionhologram directly reflects the interconnecting output light beam from aspecified logic element to at least one other logic element to form thecrossbar switch. Since cumbersome arrangements of mirrors and lenses arenot required, optical logic systems and optical signal processing can besimplified substantially. Furthermore, the optical logic elements may beoptically interconnected in a relatively small space. Since intersectinglight beams do not interfere with one another, this optical logicarrangement significantly enhances the practical use of optical parallelprocessing techniques.

Another feature of this invention is that the reflection holograms canbe made to feed back any number of interconnecting light beams to form asequential optical logic circuit.

Another advantage of this invention is that the reflection hologramsfunction with either coherent light or incoherent light from inexpensivelow-power light sources. This significantly reduces the cost of anoptical logic system as compared to a transmission hologram opticallogic system that uses only coherent light from normally higher costlaser light sources.

In accordance with still another feature, the arrangement furthercomprises director means such as a rainbow transmission hologram and aplanar mirror for directing control light beams between the opticallogic element array and other optical sources and receivers.

In accordance with yet another feature of this invention, with the useof optically nonlinear optical devices that receive optical signals oneither side of a device array, combinational and sequential opticallogic circuits can be easily optically interconnected in a relativelysmall space to form even larger and more complex optical logic circuitsand systems such as an optical digital processor.

In accordance with still yet another feature of this invention, atransparent material maintains the positions of the optical logicelements relative to the reflection hologram during formation of thereflection hologram and during use of the optical logic arrangement.

BRIEF DESCRIPTION OF THE DRAWING

The invention may be better understood from the following detaileddescription when read with reference to the drawing in which:

FIG. 1a depicts an illustrative arrangement having a plurality ofreflection holograms and optically nonlinear optical devices forperforming an optical logic operation;

FIG. 1b shows the illustrative arrangement of FIG. 1a with a rainbowtransmission hologram and a planar mirror for directing external opticalsignals to and from the device array, respectively;

FIG. 2 depicts a diagram of an illustrative optically nonlinear opticaldevice known as the self electro-optic effect device (SEED) that may beoperated to function as an optical logic NOR gate;

FIG. 3a graphically depicts the theoretical output power of the SEED ofFIG. 2 as a function of optical input power in the bistable region ofthe device;

FIG. 3b graphically responsivity S(V) of an exemplary diode structurethat may be used for the SEED of FIG. 2;

FIG. 4a graphically depicts the entire theoretical input-output powercharacteristics of the SEED of FIG. 2;

FIG. 4b graphically depicts the empirical input-output powercharacteristics and bistable switching operation of a sample SEED;

FIG. 5a depicts an illustrative arrangement for forming optical fringepatterns in a photographic emulsion to optically interconnect an arrayof optically nonlinear optical devices;

FIG. 5b depicts an illustrative arrangement for generating and directingcoherent light beams to form the optical fringe patterns of FIG. 5a;

FIG. 6 depicts an illustrative logic diagram of a well-known two-by-twocrossbar switch comprised of only NOR gates;

FIG. 7 is a nodal diagram of the crossbar switch of FIG. 6 positioned ina four-by-four array;

FIG. 8 depicts an illustrative two-by-two optical crossbar switchcorresponding to the crossbar switch of FIG. 6;

FIG. 9 depicts a rear pictorial view of the illustrative interconnectionarrangement of FIG. 1a showing selected optical signals as plane waves;

FIG. 10 shows an illustrative logic diagram of a well-known clocked JKflip-flop comprised of only logic NOR gates;

FIG. 11 is a nodal diagram of the clocked JK flip-flop of FIG. 10positioned in a three-by-three array;

FIG. 12 depicts an illustrative optical sequential logic arrangementcomprising optically nonlinear self electro-optic effect devices forimplementing an optical version of the clocked JK flip-flop of FIG. 10;

FIGS. 13a-13c illustrate the operation of a Fabry-Perot Interferometer;and

FIGS. 14a-14b illustrate the operation of a nonlinear Fabry-PerotInterferometer.

DETAILED DESCRIPTION

Depicted in FIG. 1a is an illustrative arrangement for opticallyinterconnecting a plurality 120 of optically nonlinear optical devices121 through 129 positioned in a two-dimensional array. As known in theart, the optical output signal of an optically nonlinear optical deviceis a nonlinear gain function of at least one optical input signalapplied to the device. Consequently, an optically nonlinear opticaldevice can regenerate optical signals and perform optical logicfunctions. Bias light beams 150 and 151 optically bias respectiveoptically nonlinear devices 121 and 123 in a well-known manner tofunction as optical logic elements such as optical NOR gates. Theoptical interconnection arrangement comprises reflection hologram 100or, more particularly, a plurality of reflection subholograms 101through 109 similarly positioned in a two-dimensional array. In responseto an interconnecting optical output signal such as interconnectingoutput light beam 152 received in a predetermined direction from therear surface 131 of a specified device such as 121, the correspondinglypositioned subhologram such as 101 originates one or more individualinterconnecting control light beams such as 154 and 155 each in adifferent predetermined direction back to the rear surface of one ormore other devices such as 123 and 129 in the device array. To opticallyinterconnect three or more optical devices such as 121, 123, and 128 inseries, the position of each emitting device such as 121 and 123 ismaintained in a fixed position relative to its correspondinglypositioned subhologram such as 101 and 103, respectively. Otherwise, twooptically interconnected devices need only be positioned in a fixeddirection from the correspondingly positioned subhologram. Opticallynonlinear optical devices 121 through 129 have front and rear surfaces130 and 131 for receiving optical signals propagating in either one oftwo generally opposing directions. Thus, a plurality of holograms suchas subholograms 101 through 109 can be made to interconnect thesetwo-surfaced optically nonlinear optical devices to form anycombinational or sequential optical logic circuit.

As suggested, each optically nonlinear optical device in plurality 120is responsive to light beams received on either one or both of front andrear surfaces 130 and 131 for emitting an interconnecting output lightbeam from one of the two surfaces. The wavelength of these opticalsignals or light beams can range from the ultraviolet to the infrared.Several devices such as the nonlinear Fabry-Perot Interferometer and theself electro-optic effect device (hereinafter referred to as SEED) thatwill be described hereinafter are suitable for use as the opticallynonlinear optical device. In addition, each of these devices may beoperated to function as an optical logic element such as an optical NORgate.

Each optically nonlinear device in plurality 120 is in either one of twotransmission states depending on the power of the incident light. Whenthe power of an incident light beam is below a predetermined thresholdlevel, the device is in a first transmission state and simply passes or,more particularly, receives the incident light beam and emits an outputlight beam. For example, a source of either coherent or incoherent light(not shown) illuminates the front surface 130 of plurality 120 withoptical bias signals such as bias light beams 150 and 151. The biasbeams cause respective devices 121 and 123 to function as optical logicNOR gates. When the power of bias beam 150 is just below the thresholdlevel of NOR gate 121, the gate passes bias beam 150 as a high logiclevel output light beam 152 to reflection subhologram 101. Similarly,NOR gate 123 passes bias light beam 151 as a PG,11 high logic leveloutput light beam 153 to reflection subhologram 103.

In contrast to transmission holograms, reflection holograms reflect,rather than pass, optical signals. In response to interconnecting outputbeam 152, reflection subhologram 101 originates interconnecting controllight beams 154 and 155 to respective devices 123 and 129.Macroscopically, reflection subhologram 101 originates interconnectingcontrol beams 154 and 155 by splitting and reflecting output beam 152.As a result, reflection subhologram 101 optically connects nonlinearoptical device 121 to devices 123 and 129. Since the front surface 110of reflection subhologram 101 and the rear surface 131 of device 121 arepositioned in a substantially parallel manner directly facing eachother, subhologram 101 receives interconnecting output beam 152 in adirection having a zero degree angle of incidence 170 with respect tothe normal 169 of front surface 110. The direction of interconnectingoutput light beam 152 would change, and the incident angle would, ofcourse, increase if the two opposing surfaces of subhologram 101 anddevice 121 were either not parallel or did not directly face each other.

A reflection hologram can originate any number of interconnectingcontrol light beams and originate each one in a different direction.Thus, as illustrated, control light beams 154 and 155 propagate indifferent directions to respective devices 123 and 129 and in adirection generally opposite to that of interconnecting output beam 152.The total optical power of the reflected interconnecting control beams154 and 155 will, of course, be somewhat less than the power of theinterconnecting output light beam 152.

Similarly, reflection subhologram 103 reflects interconnecting outputbeam 153 received from optical NOR gate 123 as interconnecting controlbeam 156 to optically nonlinear device 128. Without a bias beam incidenton optically nonlinear devices 128 and 129, a low-power levelinterconnecting control beam normally passes straight through thedevice.

When bias beam 151 and high logic level interconnecting control beam 154are both incident on NOR gate 123, the power of the incident light beamsexceeds the predetermined threshold level of the device and causes thedevice to assume a second transmission state. In this second state, thedevice either absorbs a significant portion of the incident light orreflects the incident light depending on the type of device used. As aresult, interconnecting output beam 153 from NOR gate 123 and reflectedinterconnecting control beam 156 from reflection subhologram 103 assumea low logic level.

In addition, optical control signals other than those from hologram 100such as a high logic level control beam from an independent opticalsource such as another optical logic circuit (not shown) may be used tocontrol the state of any device in the array. For instance, a pluralityof optical control signals transmitted via a fiber optic bundle can beindividually directed to specific array devices by, for example, awell-known rainbow transmission hologram. To further illustrate thispoint, FIG. 1b shows another view of the illustrative opticalinterconnection arrangement of FIG. 1a with rainbow transmissionhologram 136 for directing control beam 157 from an independent opticalcontrol source to optically nonlinear device 121.

Well-known rainbow transmission hologram 136 is maintained in a positionperpendicular to surface 130 of device array 120 by any suitabletransparent material 132 such as silicon glass or sapphire. Thetransparent material maintains the rainbow hologram and device array ina fixed perpendicular position and readily passes bias beam 150 andcontrol beam 157 to reach optical receiving and emitting area 114 ofdevice 121. One of the fibers in bundle 133, which is attached to therainbow hologram in a suitable manner, guides the control beam from theindependent optical control source to the rainbow hologram. Rainbowhologram 136 then redirects or, more particularly, passes the controlbeam therethrough to device 121. By way of example, when high logiclevel control beam 157 and optical bias beam 150 are both incident onfront surface 130 of the array, device 121 absorbs the incident light,and interconnecting output beam 152 along with associatedinterconnecting control beams 154 and 155 assume a low logic level.Again, depending on the type of optical device used, a low logic leveloptical signal may be either the absence of light or an optical signalsignificantly attenuated with respact to a high logic level opticalsignal.

Control light beams from the device array to an independent opticalreceiver are emitted by, again, simply passing a control beam from areflection subhologram through an optically nonlinear device in thearray when a bias beam is not incident on the device. As shown,optically nonlinear device 129 passes interconnecting control beam 155from reflection subhologram 101 to an independent optical receiver suchas another optical logic circuit. Likewise, optically nonlinear device128 passes interconnecting control beam 156 from reflection subhologram103 to the independent receiver. In addition, light beams from the arraymay be directed to the independent optical receiver by the use of aplanar mirror 134 and another optical fiber bundle 135 as shown in FIG.1b. The transparent material positions and maintains the mirror withrespect to nonlinear devices 128 and 129 so that it directs or, moreparticularly, reflects each of light beams 155 and 156 to a particularfiber in the bundle. The bundle is, again, affixed to the transparentmaterial in a suitable manner.

Only a single line was utilized in FIG. 1a and 1b to depict each oflight beams 150 through 157. However, each optical signal approximates aplane wave or, more particularly, either a very slightly diverging orconverging spherical wave, depending on the direction in which the waveis traveling with respect to the device. Depicted in FIG. 9 is a rearpictorial view of the interconnection arrangement of FIG. 1aillustrating each one of light beams 150-152, 154 and 155 as a planewave. Recall that reflection subhologram 101 optically interconnectsoptically nonlinear devices 121, 123, and 129.

In this illustrative embodiment, the optical receiving and emittingareas of each optically nonlinear device have been formed into arectang1e. As shown in FIG. 9, when cylindrically-shaped bias beam 150is received, the front surface 130 of device 121 is illuminated. Ofcourse, only the rectangular receiving and emitting area 114 of device121 is responsive to the cylindrically-shaped bias beam 150. Whenemitted from the rectangular receiving and emitting area 115 of rearsurface 131 of device 121, interconnecting output beam 152 approximatesa rectangular-shaped plane wave that diverges slightly in a well-knownmanner to illuminate a predetermined area of front surface 110 ofreflection subhologram 101. In response to output beam 152, subhologram101 originates rectangularly-shaped interconnecting control beams 154and 155 that converge slightly on optically nonlinear devices 123 and129, respectively. Interconnecting control beams 154 and 155 converge inthe opposite manner that the coherent light beams previously divergedfrom devices 123 and 129 in forming subhologram 101. This divergence issimilar to that of interconnecting output light beam 152. Reflectionsubhologram 101 reflects interconnecting control beams 154 and 155 fromthe same area of front surface 110.

Since a reflection hologram can be made to reflect an optical signal inany one or more of a plurality of predetermined directions, reflectionhologram 100 can be made to optically interconnect the devices of thearray in any desired combination. Furthermore, since only logic NORgates are needed to form any combinational or sequential logic circuitor any combination thereof, the reflection hologram may be made tooptically interconnect an array of optical logic NOR gates to form anydesired combinational or sequential optical logic circuit that utilizesall optical information signals. Not only does this opticalinterconnection arrangement facilitate serial data processing but, inaddition, facilitates parallel processing in which large numbers ofparallel-connected optical logic elements may be accessed at the sametime.

Depicted in FIG. 2 is a diagram of one illustrative optically nonlinearoptical device, which may be used for devices 121 through 129,comprising a multiquantum well (MQW) structure known as a selfelectro-optic effect device (SEED). The SEED functions as a modulatorand photodetector and may be operated to function as an logic elementsuch as an optical logic NOR gate. The SEED requires very littleswitching energy with respect to other bistable devices such as thenonlinear Fabry-Perot Interferometer, which may also be used for devices121 through 129. Empirically, the optical switching energy of the SEEDwas found to be approximately 4 femtojoules/square micron, and the totalswitching energy including electrical energy was found to beapproximately 20 femtojoules/square micron. A SEED having a largephotosensitive area of 28,000 square microns was found to have switchingenergy of approximately 1.0-1.5 nanojoules. SEEDs having smallerphotosensitive areas will, of course, have lower switching energies andfaster operation. The self electro-optic effect device is described inan article by D. A. B. Miller et al., entitled "A Novel Hybrid OpticallyBistable Switch: The Quantum Well Self Electro-Optic Effect Device",Applied Physics Letters. Volume 44, No. 1, July 1, 1984. This device isalso described by D. A. B. Miller in U.S. patent application Ser. No.589,556 filed Mar. 14, 1984, that description being herein incorporatedby reference. However, the SEED will be briefly described herein so asto enable the reader to better understand the operation of the SEED withrespect to FIGS. 2 through 4b.

As shown in FIG. 2, the SEED includes a layered multi-quantum well (MQW)201 in the intrinsic (i) region of reverse-biasedpositive-intrinsic-negative (p-i-n) diode structure 202. A multi-quantumwell structure has a plurality of thin, narrow bandgap layersinterleaved with a plurality of thin, wide bandgap layers. The narrowbandgap layers are sufficiently thin that quantum effects are evident,and important, in the carrier energy levels. With photon sources having0.8-0.9 micron wavelengths, the layers may be comprised of well-knowncompounds such as AlGaAs/GaAs. However, other group 3,5 compounds suchas InGaAsP and InGaAs/AlIs may be used with photo sources having 1.3-1.5micron wavelengths. This multi-quantum well diode structure is describedin detail in an article by T. H. Wood et al., entitled "High-SpeedOptical Modulation with GaAs/GaAlAs Quantum Wells in a P-I-N DiodeStructure", Applied Physics Letters, Volume 44, No. 1, Jan. 1, 1984, atpage 16. When an electric field is perpendicularly applied to the layersof the structure, i.e., parallel to the small dimension of the layers,the absorption band edge including any exciton resonance peaks, can beshifted to lower photon energies. In only microns of material, changesin optical absorption of 50 percent can be readily achieved. Theseoptical absorption changes may be used to make an optical modulator thatoperates at room temperature and is compatible with present laser diodepowers, wavelengths, and materials. The absorption is greatly enhancedrelative to the shifts seen in bulk materials because of carrierconfinement in the quantum wells. AlGaAs multi-quantum wells also showexceptionally strong room-temperature exciton resonances, which enhancethe absorption effects at the band edge. Furthermore, the use of a p-i-ndoping scheme in this structure allows the application of a moderatelylarge electric field to the active layers without high voltage orcurrent drive. The p and n layers must have bandgaps so that theselayers do not absorb the incident light. Applying this electric field toa reverse-biased diode structure, the structure is also an efficientphotodetector.

The principles of optical absorption exhibited by the SEED are: first,that increasing the intensity of an input light beam increases theoptical absorption coefficient of the device; and second, thatincreasing the absorption of light energy by the device also increasesthe optical absorption coefficient. As a consequence of these twoprinciples, the SEED switches nonlinearly between two optical states oftransmission. This will be better understood from the followingdescription of device operation.

Constant bias voltage supply 203 and positive feedback resistor 204 areserially connected to p-i-n diode structure 202 to form the opticallynonlinear self electro-optic effect device. To make the SEED switch, theincident wavelength is chosen to be near the exciton resonance positionfor zero voltage across the diode. With a low power level light beam 250incident on the multi-quantum well diode structure, nearly all thevoltage of constant potential 203 is applied across the diode structure,as there is very little, if any, photocurrent in the circuit.

Increasing the optical input power increases the photocurrent, therebyincreasing the voltage drop across resistor 204 and reducing the voltageacross the diode. This reduced voltage causes increased opticalabsorption as the exciton resonances move back, resulting in furtherincreased photocurrent and consequently leading to regenerative feedbackand switching.

The theoretical optical output power of the SEED of FIG. 2 undervariations in power of an input light beam such as 250 is graphicallyshown in FIG. 3a. Note that, in the following discussion, the units ofbeam power are used rather than intensity. Beam power is expressed inunits of watts and beam intensity in units of watts per square meter.Beam power is the integral of intensity over a cross sectionperpendicular to the axis of the light beam. Units of power are moreuseful in the following discussion because the SEED responds to beampower and, more particularly, to absorbed beam power. In FIG. 3a, thepower of the input light beam is plotted along the horizontal axis.Along the vertical axis the power of the output light beam such as 251of FIG. 2 is plotted as output power. At an input power of value A, theoutput power is given by a value of TA. As the input power is increasedto a value of B, the output power increases to a value of TB1. However,the absorption coefficient of the SEED increases with increasing inputpower, and therefore the transmission curve 320 from input power A toinput power B is below a line of constant slope such as straight line321. A further increase of input power from value B to value C resultsin a further increase in the absorption coefficient so that the SEEDbecomes unstable and switches from a value of TC1 to a value of TC2. Afurther increase of input power to a value of D leads to an output powervalue of TD.

Decreasing the power of the input light beam from input power level Cresults in the output power tracing along curve 320 to input power B,whereupon the absorption coefficient of the SEED switches to a smallervalue, and the output power increases from TB2 to the value TB1. TheSEED is said to exhibit optical bistability because it switches from astate of high transmission to a state of low transmission as the opticalinput power is increased, and switches back to a state of hightransmission as the optical input power is decreased. However, opticalbistability may exist for other sequences of switching.

The empirical responsivity S(V) of an exemplary p-i-n diode structurethat may be utilized for diode 202 is depicted as curve 330 in FIG. 3b.The responsivity S(V) is the photocurrent produced per unit of incidentlight power, and is expressed in the units of amperes per watt (A/W) ofincident light power. Curve 330 shows the measured (external)responsivity S(V) of the exemplary diode structure as a function ofreverse bias, this measurement being made with a laser light sourcetuned to a photon energy of 1.456 eV (851.7 nm). This photon energy isapproximately the heavy hole resonance energy at conditions of zero biasfor the MQW structure used. As the reverse bias is increased, theresponsivity first increases as photocurrent collection becomescomplete, and then decreases as the exciton absorption peak moves tolower energy. The "bump" between 8 V and 16 V in curve 330 is due to thelight hole exciton resonance similarly moving past the measuringwavelength.

The input-output power characteristic of the exemplary diode structurewhen connected in series with a positive feedback resistor and apositive source of potential, as shown in FIG. 2, may be calculated bysolving two simultaneous equations. The first equation involves themeasured responsivity S(V) of the MQW diode structure as shown in FIG.3b by curve 330 where;

    S=S(V)                                                     (1).

The second equation is V=V_(o) -RSP where P is the optical input power,R is the resistance of the feedback resistor, V_(o) is the voltage ofthe constant bias voltage supply, and V is the voltage across the diode.This equation may also be written as: ##EQU1## The graphical solution isstraightforward with equation 2 giving dashed straight lines A, B, C,and D in FIG. 3b of decreasing negative slope for increasing P.Bistability results from the multiple intersections of a straight lineand curve 330. Straight lines A and D intersect curve 330 only once.Straight lines B and C have tangent points 331 and 332 with curve 330.The tangent points 331 and 332 represent unstable switching points. Allstraight lines between lines B and C have three intersection points withcurve 330, the middle intersection point representing unstableoperation.

The responsivity S and voltage V across the diode may also be calculatedas a function of P by choosing V, deducing S from equation 1, and P fromequation 2. For reverse bias equal to or greater than 2 volts, opticalabsorption closely follows the responsivity. However, to make a moreaccurate calculation, the output power P_(out) (≈PT ) for each value ofdiode voltage V and optical input power P can be deduced from themeasured transmission T(V). Hence, the whole theoretical input-outputpower characteristic of a sample SEED as graphically depicted in FIG. 4amay be derived.

The empirical input-output power characteristic 410 of a sample SEED isshown in FIG. 4b. The optical output power of the device increases alongcurve 410 until the input power reaches predetermined input thresholdlevel 401. At this level, the positive feedback causes the device tobecome unstable, and the optical output power rapidly switches fromoutput level 402 to lower output level 403. At this lower output powerlevel, the diode structure absorbs approximately half of the incidentlight. Further increasing the incident light power above input level 401causes the output power level to again increase but at a rate less thanthe prior rate. Reducing the incident light power below input level 401will accordingly decrease the output power of the device. Thus, atsecond predetermined input power level 404, the device once againbecomes unstable, and the optical output power switches to a higheroutput power level on curve 410.

With this bistable optical output condition, it should be readilyapparent that the self electro-optic effect device (SEED) can beoptically biased to function as an optical logic NOR gate. As shown inFIG. 4b, a constant optical bias signal such as a light beam that hasincident input power level 405 just below predetermined input thresholdlevel 404 optically biases the SEED to a point below the knee 406 oninput-output power characteristic curve 410. With just the bias beamincident on the device, the optical absorption of the diode structure islow, and most of the bias beam is consequently passed therethrough. Whenadditional light such as a control light beam is incident on the device,the combined incident optical power is greater than input thresholdlevel 401, and the optical output power of the device switches to alower output level such as 403. Since any one of a number of opticalcontrol signals in combination with the bias signal can cause theoptical output power of the device to switch to a lower transmissionlevel, the SEED functions as an optical logic NOR gate. When all opticalcontrol signals are extinguished, only the optical bias signal isincident on the device, and the optical output signal returns to ahigher output power level on the curve.

Although the sample device exhibits a hysteresis-like operation betweenthe two stable transmission states, the hysteresis-like effect may beminimized by the use of a light source having photon energies assuggested and described in an article by D. A. B. Miller et al.,entitled "Optical Bistability Due to Increasing Absorption", OpticsLetters. Vol. 9, No. 5, May 1984, at page 162. The hysteresis-likeoperation is also a function of the constant bias voltage supply V₀ andcan be also minimized by selecting V₀ so as to intersect theresponsivity curve 330 as shown in FIG. 3b only once over the operatingrange of the device. In addition, it is reasonably expected that opticalabsorption in excess of 80 percent will be readily obtainable.

Other bistable optically nonlinear optical devices such as the nonlinearFabry-Perot Interferometer may also be used for optically nonlineardevices 121 through 129. The nonlinear Fabry-Perot Interferometerreflects rather than absorbs incident light. Having switching speedscomparable to the SEED, the nonlinear Fabry-Perot Interferometerrequires significantly higher switching power with the optical signalssupplying all the power. Like the SEED, the nonlinear Fabry-PerotInterferometer may be optically biased to function as an optical logicelement such as an optical NOR gate. Furthermore, the nonlinearFabry-Perot Interferometer receives incident optical control signals oneither of the two major array surfaces and emits an optical outputsignal from one of the same two surfaces. However as suggested, theswitching power and energy of the nonlinear interferometer areconsiderably higher than those of the SEED. In addition, the nonlinearinterferometer responds only to coherent light. A detailed descriptionof the nonlinear Fabry-Perot Interferometer is described by D. A. B.Miller in an article entitled, "Refractive Fabry-Perot Bistability withLinear Absorption: Theory of Operation and Cavity Optimization", IEEEJournal of Ouantum Electronics, Vol. QE-17, No. 3, March 1981. Anotherdescription of the nonlinear Fabry-Perot Interferometer for use as anoptical logic element is described by J. L. Jewell, et al. in an articleentitled, "Use of a Single Nonlinear Fabry-Perot Etalon as Optical LogicGates", Aplied Physics Letters. Vol. 44, No. 2, Jan. 15, 1984, at page172.

FIGS. 13a through 13c illustrate the operation of a linear Fabry-PerotInterferometer 1301. Basica:lly, the interferometer comprises a cavity1302 with front and rear walls 1303 and 1304. For example in the generalcase illustrated in FIG. 13a, 90% of incident light beam 1350 at frontwall 1303 is reflected as reflected output beam 1351. The remaining 10%of the incident beam is refracted as forward beam 1352. The refractedforward beam (10%) that enters the cavity is again divided with 90%being reflected as reverse beam 1353 (9%) and 10% again being refractedas transmitted output beam 1354 (1%). In FIG. 13b, the length of thecavity is such that the forward and the reverse beams destructivelyinterfere to form resulting wave 1355. The power of the transmittedoutput beam in this case is less than one percent of the incident lightpower. In FIG. 13c, the length of the cavity is such that the forwardand the reverse beams constructively interfere to form resulting wave1356. Here, the power of the transmitted output beam is nearly 100% ofthe incident light beam with little, if any, power being reflected fromfront wall 1303.

In FIG. 14a, the operation of a nonlinear Fabry-Perot Interferometer1401 is illustrated. Here, the cavity 1402 of the interferometer isfilled with a nonlinear material 1405. The index of refraction of thisnonlinear material and consequently the optical path length of thematerial in the cavity varies as a function of the incident light power.Utilizing this property, the optical path length of the nonlinearinterferometer is chosen such that little, if any, optical power istransmitted when the power of the incident light is below a thresholdlevel. When the incident light power exceeds the threshold level, theindex of refraction and the optical path length of nonlinear material1405 change to transmit nearly 100% of the incident light power. Likethe linear interferometer depicted in FIGS. 13a through 13c, there aretwo output beams from the nonlinear interferometer that are of interest:a reflected output beam 1452 and a transmitted output beam 1453.

In FIG. 14b, exemplary input-output optical power curve 1410 graphicallyillustrates the optical output power of nonlinear interferometer 1401 asa function of the optical input power. This input-output curve will beused to illustrate how a nonlinear interferometer may be operated toperform an optical logic NOR function. Returning to consideration ofFIG. 14a, an optical bias beam 1450 with input power level 1460 biasesthe nonlinear interferometer to a point on its input-output curve asillustrated in FIG. 14b that will cause reflected output beam 1452 torapidly change from high logic output power level 1461 to low logicoutput power level 1462. The additional power to raise input power level1460 to level 1463 is provided by high logic level control light beam1451. When high logic input power level control beam 1451 and bias beam1450 are incident on the interferometer, reflected output beam 1450assumes low logic output power level 1462. When bias beam 1450 and onlylow logic input power level control beams are incident on the device,the reflected output beam is at high logic output power level 1461.Thus, nonlinear interferometer 1401 operates as an optical logic NORgate. In addition, transmitted output beam 1453 may be utilized so thatthe nonlinear interferometer performs an optical logic OR function.

Still another optically nonlinear device is the liquid crystal lightvalve that can only receive optical control signals on a front surfaceand emit an optical output signal from the rear surface. Such a devicecould be used to implement the present invention. However, appropriateapparatus would have to be positioned so as to direct the optical outputsignal to the reflection hologram and then onto the front surface of thevalve.

The reflection hologram of the present invention optically interconnectsnonlinear optical devices and, in particular, optically nonlinearoptical devices that are operated to function as optical logic NORgates. A plurality of reflection holograms may be made to interconnectthese optical logic gates optically to form any desired optical logiccircuit.

A detailed description of reflection holograms is discussed by H. J.Caulfield, editor, in the Handbook of Optical Holography, AcademicPress, 1979, and by Collier et al. in Optical Holography. AcademicPress, 1971. In addition, a series of articles edited by T. H. Jeong formaking reflection holograms may be found in the Proceedings of theInternational Symposium on Display Art Holography, Holography Workshops,Lake Forest College, Lake Forest, Ill., 1982. Making reflectionholograms is generally well known in the art. However, a description ofhow to make a reflection hologram of the present invention such asreflection hologram 101 for optically interconnecting nonlinear opticaldevices will be given next.

Depicted in FIG. 5a is an illustrative arrangement for forming andstoring optical fringe patterns in a photosensitive material such as aphotographic emulsion for optically connecting nonlinear optical device121 to devices 123 and 129. An unexposed photographic emulsion 500 and aplurality of optically nonlinear optical devices 121 through 129 areaffixed in a well-known manner to the parallel surfaces 503 and 504 ofoptically transparent material 505. For example, the opticallytransparent material may be silicon glass or, preferably, a high thermalconductivity material such as sapphire. The optically transparentmaterial maintains the nonlinear devices positioned in a two-dimensionalarray and photographic emulsion 500 in a fixed relative parallelposition during both exposure of the photographic emulsion and afterprocessing when the fringe patterns have been permanently stored in theemulsion. The index of refraction of the transparent material isselected to reduce Fresnel reflections. However, as suggested, asapphire material is preferred because it more evenly distributes theheat generated by the optical signals passing therethrough.

Also included in the recording arrangement are spatial light modulatormasks 501 and 502 such as well-known electrically operated magneto-opticarrays that are temporarily affixed to rear surface 111 of the unexposedphotographic emulsion and front surface 130 of the device array,respectively. As shown in FIG. 5a, coherent light beam 551 passesthrough mask 501 along with coherent light beams 552 and 553 throughmask 502 to illuminate outside surface 111 of photographic emulsion 500and outside surface 130 of devices 123 and 129, respectively.

FIG. 5b depicts the derivation of coherent light beams 551 through 553from a single coherent beam. Coherent light beam 570 from a coherentlight source 520 such as a commercially available laser is split intotwo coherent light beams 551 and 571 by commercially available variablebeam splitter 521. One or more light beam directors such as mirrors 522and 523 are positioned to direct coherent reference beam 551 toilluminate a predetermined area of emulsion 500 designated forreflection subhologram 101. Coherent beam 571 is split into twoequal-power coherent object light beams 552 and 553 by another beamsplitter 524. In this illustrative embodiment, object beams 552 and 553have half the optical power of reference beam 551 and are directed toilluminate front surface 130 of respective devices 123 and 129 by one ormore light beam directors such as beam splitter 524 and mirror 525.However, the power of the coherent object beams 552 and 553 may bedifferent to vary proportionately the power of interconnecting controllight beams 153 and 154, respectively.

As shown in FIG. 5a, reference light beam 551 is directed through aportion of rear surface 111 of the photographic emulsion that is exposedby spatial light modulator mask 501 to illuminate receiving and emittingsurface 115 of device 121. Similarly, object beams 552 and 553 aredirected through front surface 130 of respective devices 123 and 129that are exposed by spatial light modulator mask 502 to interfere withreference beam 551 in the photographic emulsion. The difference indistance traveled between coherent reference light beam 551 and each ofcoherent object light beams 552 and 553 from the coherent light sourceto the photographic emulsion should be much less than the coherencelength of the beams.

To form optical fringe patterns in the emulsion for reflectionsubhologram 101, mask 501 is electrically operated in a well-knownmanner to form an opening for coherent reference light beam 551 to enterthe rear surface 111 of the emulsion. In addition, mask 502 is alsoelectrically operated to form openings for coherent object light beams552 and 553 to pass through optically nonlinear devices 123 and 129,respectively. Object beams 552 and 553 pass through respective devices123 and 129 and enter the photographic emulsion through front surface130. Reference beam 551 and object beams 552 and 553 approximate planewaves. As previously suggested, the plane wave of beams 552 and 553diverge a small amount from rear surface 131 of the devices to frontsurface 110 of emulsion 500. Object beams 552 and 553 interfere withreference beam 551 in the emulsion to form three-dimensional opticalinterference patterns or, more particularly, well-known Bragg filteroptical fringe patterns that are stored in the photographic emulsion.Similarly, each area of the photographic emulsion designated for aparticular subhologram is exposed to form a three-dimensional opticalfringe pattern in the emulsion associated with the correspondinglypositioned device. After each designated area of the photographicemulsion is separately exposed, spatial light modulator masks 501 and502 are removed. The exposed photographic emulsion is then processed ina well-known manner to permanently store the optical fringe patterns. Asshown, the originally stored optical fringe patterns are directly usedto optically interconnect devices 121 through 129. However, theoriginally stored fringe patterns may also be used as a master toreplicate copies.

The permanently stored fringe patterns form a plurality of reflectionholograms for optically interconnecting devices 121 through 129 in apredetermined manner to perform a desired logic operation. Asillustrated in FIG. 1a, interconnecting output light beam 152illuminates the front surface 110 of reflection subhologram 101 and isthe conjugate of coherent reference light beam 551 used to form thefringe pattern. Conjugate light beams propagate in directly oppositedirections with respect to each other. In response to interconnectingoutput beam 152, the optical fringe pattern of reflection subhologram101 originates interconnecting control beam 154 to device 123 andinterconnecting control beam 155 to device 129. These interconnectingcontrol light beams are the conjugates of coherent object light beams552 and 553, respectively. In effect, the optical fringe pattern ofsubhologram 101 functions as a set of optical beam splitters and mirrorsto split and reflect interconnecting output light beam 152 asinterconnecting control light beams 154 and 155 to respective devices123 and 129. Thus, it should be apparent that the reflection hologram ofthe present invention may be used in either a "space-variant" or"space-invariant" arrangement as discussed in the Sawchuk article(supra) to interconnect a plurality of optically nonlinear opticaldevices to form any combinational or sequential logic circuit.

Depicted in FIG. 6 is a logic diagram of an illustrative two-by-twocrossbar switch 600 with input terminals IN₀ and IN₁, respective outputterminals OUT₀ and OUT₁, and control terminal C. This well-known two-by-two combinational logic crossbar switch comprises NOR gates 601through 612 interconnected as shown. For illustrative purposes, thelogic NOR gates are also designated A through L so that they may bearranged on three rows of a four-by-four array and likewiseinterconnected to form the two-by-two crossbar switch.

Depicted in FIG. 7 is a nodal diagram that illustrates theinterconnection of NOR gates 601 through 612 on three rows of afour-by-four array of nodes designated A through P. The nodal diagramalso illustrates how a corresponding optical two-by-two crossbar switchmay be arranged and interconnected on a four-by-four array of opticallynonlinear optical devices. Nodes A through L represent logic gates 601through 612, respectively. Nodes A through L also represent three rowsof a four-by-four optically nonlinear optical device array, and nodes Mthrough P represent the fourth row of the optical device array. Thelines not only represent the necessary interconnections of NOR gates 601through 612 to form a crossbar switch but also the necessaryinterconnecting light beams from each of the reflection subhologramsthat are needed to form a corresponding two-by-two optical crossbarswitch. Macroscopically, each reflection subhologram in a space variantinterconnection arrangement splits and reflects the singleinterconnecting output light beam from the correspondingly positionedoptically nonlinear device into as many control beams as are needed.

As suggested, the two-by-two optical crossbar switch comprises sixteenoptically nonlinear optical devices positioned in a four-by-fourtwo-dimensional array and a plurality of reflection subholograms also infour-by-four two-dimensional array to interconnect the devices asindicated in the nodal diagram of FIG. 7. The optical switch requirestwo more devices (nodes N and P) than illustrative switch 600 to passthe reflected output beams from the rear surface of elements J and L andout the front surface.

Depicted in FIG. 8 is an illustrative two-by-two optical crossbar switch850 comprising a four-by-four array 800 of bistable self electro-opticeffect devices (SEEDs) 801 through 816 and a four-by-four array 820 ofreflection subholograms 821 through 836 for optically interconnectingthe SEED array as indicated in the nodal diagram of FIG. 7. Devices 801through 816 correspond to nodes A through P, respectively, and are sodesignated in FIG. 8. Either a coherent or incoherent light source (notshown) illuminates the front surface 817 of SEEDs 801 through 812 withbias light beams 851 through 862, respectively. These optical bias beamscause SEEDs 801 through 812 to function as optical logic NOR gates.Bistable devices 813 through 816 are shielded from the light source andeither absorb or pass optical signals depending on the incident powerthereof.

Referring to FIGS. 6 through 8, the output signals on the outputterminals OUT₀ and OUT₁ of a two-by-two crossbar switch depend on thelogic level of the control signal on the control terminal C and on thelogic level of the input signals on the corresponding input terminalsIN₀ and IN₁. In this illustrative embodiment, the switch is in the"crossed state" when a high logic level signal is applied to the controlterminal C. As a result, the logic level on input terminal IN₀ isswitched to output terminal OUT₁, and the logic level on input terminalIN₁ is switched to output terminal OUT₀. When a low logic level isapplied to the control terminal C, the switch is in the "bar state", andthe logic level or each of the input terminals IN₀ and IN₁ is switchedto the corresponding output terminals OUT₀ and OUT₁, respectively. Thiscan of course be verified with corresponding crossbar switches 600 and850 by applying a set of logic level signals to the input and controlterminals and tracing the logic levels through the NOR gates of theswitches to the output terminals.

As shown in FIG. 8, a low logic level control light beam 840L fromoptical input source terminal IN₀ is applied to optical NOR gate 801,and high logic level control beams 863H and 864H from respective inputand control source terminals IN₁ and C are applied to respective opticalNOR gates 802 and 803. Accordingly, optical crossbar switch 850 is inthe "crossed state". As a result, optical NOR gate 810 emits a highlogic level interconnecting output light beam 873H to optical outputreceiver terminal OUT₀, and optical NOR gate 812 emits a low logic levelinterconnecting output light beam 841L to optical output receiverterminal OUT₁. In order to pass high logic level interconnecting outputlight beam 873H from the front surface 817 of SEED array 800, reflectionsubhologram 830 reflects high logic level interconnecting output beam873H to SEED 814. Since a bias beam is not incident on any of SEEDs 813through 816, high logic level interconnecting output beam 873H passesthrough SEEL 814 to output receiver terminal OUT₀. Similarly, SEED 816passes low logic level interconnecting output light beam 841L fromreflection subhologram 832 to optical output receiver terminal OUT₁.

To verify the logic operation of corresponding two-by-two crossbarswitches 600 and 850 depicted in respective FIGS. 6 and 8, theaforementioned combination of input logic level control signals isapplied to the corresponding inputs of the two crossbar switches. Withrespect to optical crossbar switch 850, since bias light beam 851 isincident along with low logic level input control beam 840L on opticalNOR gate 801, the bias beam is passed through the gate and emitted as ahigh logic level interconnecting output beam 865H to reflectionsubhologram 821. Reflection subhologram 821 splits and reflects outputbeam 865H as high logic level interconnecting control beams 866H and867H to respective optical NOR gates 805 and 808.

To minimize the possible confusion created by a large number ofintersecting lines in FIG. 8, an array of dots on each of reflectionsubholograms 821 through 832 represents the relative center position ofSEEDs 801 through 816. A shaded box positioned over the relative centerposition of a optical logic element on a subhologram merely illustratesto what logic element an output beam is reflected. For example,subhologram 821 has two shaded boxes, one over the relative centerposition of logic element 805 and the other over element 808. Thus, aninterconnecting output light beam from logic element 801 is split andreflected to elements 805 and 808.

When a high logic level optical control signal in addition to theoptical bias signal is incident on an optical logic NOR gate, theoptical NOR gate absorbs most of the incident light as previouslydescribed, and a low logic level optical output signal is emitted to thecorresponding reflection subhologram. Such is the case with optical NORgates 802 and 803 emitting respective low logic level interconnectingoutput beams 843L and 844L when respective high logic level inputcontrol beams 863H and 864H are incident thereon. Reflection subhologram822 splits and reflects interconnecting output light beam 843L as lowlogic level interconnecting control beams 845L and 846L to respectiveoptical NOR gates 806 and 807. Similarly, reflection subhologram 823splits and reflects interconnecting output beam 844L as low logic levelinterconnecting control beams 847L, 848L, and 849L to respective NORgates 804, 806, and 808.

This combination of crossbar switch input control signals is alsoillustrated in FIG. 6 when a low logic level input signal 640L frominput terminal IN₀ is applied to single input NOR gate 601. The singleinput gate inverts the low level logic input signal and sends high logiclevel control signals 666H and 667H to an input of respective NOR gates605 and 608. Applying a high logic level input control signal 663H frominput terminal IN₁ to single input NOR gate 602, the gate inverts thehigh logic level signal and sends low logic level control signals 645Land 646L to respective NOR gates 606 and 607. Similarly, applying a highlogic level control signal 664H from control terminal C to single inputNOR gate 603, the gate sends low logic level control signals 647L, 648L,and 649L to respective NOR gates 604, 606, and 608.

Single input NOR gate 604 inverts low logic level signal 647L and sendshigh logic level control signals 669H and 670H to an input of respectiveNOR gates 605 and 607. This is similarly depicted in FIG. 8 with lowlogic level interconnecting control beam 847L incident on the rearsurface 818 of optical NOR gate 804. As a result, the gate passes biasbeam 854 to emit high logic level interconnecting output beam 868H toreflection subhologram 824. Reflection subhologram 824 splits andreflects interconnecting output beam 868H as interconnecting controlbeams 869H and 870H to respective optical NOR gates 805 and 807.

Again, applying a high logic level signal to any input of a NOR gatecauses the output signal therefrom to assume a low logic level. This isthe case with NOR gates 605, 607, and 608 in FIG. 6 and correspondingoptical NOR gates 805, 807, and 808 in FIG. 8. However, applying a lowlogic level signal to all the inputs of a NOR gate causes the gateoutput signal to assume a high logic level. As depicted in FIG. 6 withlow logic level signals (L) on all of the inputs of NOR gates 606 and611, the output signals from each assumes a high logic level (H). With ahigh logic level signal now on an input of NOR gate 609, the outputsignal assumes a low logic level (L). This is illustrated in FIG. 8 byoptical NOR gates 806 and 811 passing respective bias beams 856 and 861as corresponding high logic level interconnecting output beams 871H and875H, respectively. Reflection subhologram 826 reflects output beam 871Has a high logic level interconnecting control beam 876H to optical NORgate 809. Whereas, reflection subhologram 831 reflects interconnectingoutput beam 875H as a high logic level interconnecting control beam 872Hto optical NOR gate 812.

With high logic level interconnecting control beam 872H incidentthereon, optical NOR gate 812 absorbs most of the incident light andemits a low logic level interconnecting output beam 841L to reflectionsubhologram 832. The reflection subhologram reflects interconnectingoutput beam 841L through unbiased SEED 816 to optical output receiverterminal OUT₁. This is illustrated in FIG. 6 by applying a high logiclevel signal to single input NOR gate 612 whose output signal onterminal OUT₁ of the crossbar switch assumes a low logic level. Applyinga low logic level signal to single input NOR gate 610 results in a highlogic level output signal (H) being present on output terminal OUT₀ ofthe crossbar switch. This is likewise illustrated in FIG. 8 by opticalNOR gate 810 passing bias beam 860 as a high logic level interconnectingoutput beam 873H as previously described. Reflection subhologram 830then reflects this output beam through unbiased SEED 814 to opticaloutput receiver terminal OUT₀.

Briefly summarizing the operation of the crossbar switch in the "crossedstate", when a high logic level signal is applied to the controlterminal C, the logic level of the signal on each of input terminals IN₀and IN₁ is switched to the opposite output terminal OUT₁ and OUT₀,respectively. Similarly, the operation of crossbar switches 600 and 850can be readily verified when the switch is in the "bar state". That is,when a low logic level signal is applied to the control terminal C, thelogic level of each of the signals on input terminals IN₀ and IN₁ isswitched to the corresponding output terminal OUT₀ and OUT₁,respectively.

Not only can reflection holograms interconnect optically nonlinearoptical devices to form combinational logic circuits such as opticalcrossbar switches, but an array of reflection subholograms may also beused to interconnect an array of optically nonlinear devices to form anysequential logic circuit. In contrast to a combinational logic circuit,a sequential logic circuit typically includes at least one feedback pathfor providing various timing and memory functions that are performed,for example, in a digital processor. By way of example, a logic diagramof a well-known clocked JK flip-flop logic circuit 1000 is depicted inFIG. 10. Being a combination of combinational and sequential opticallogic circuits, a clocked JK flip-flop logic circuit may also beconsidered as a very basic digital processor. The flip-flop circuitcomprises only logic NOR gates such as 1001 through 1006 interconnectedas shown. In contrast to illustrative two-by-two combinational logicswitch 600, clocked JK flip-flop circuit 1000 includes four separatefeedback paths. In particular, the output of NOR gate 1006 is fed backto one input of NOR gate 1007, and similarly, the output of NOR gate1007 is fed back to one input of NOR gate 1006. The third feedback pathincludes feeding back the output signal of NOR gate 1007 to an input ofNOR gate 1004, and the last feedback path includes connecting the outputof NOR gate 1006 to an input of NOR gate 1005.

In a manner similar to that used for combinational logic circuit 600,clocked JK flip-flop circuit 1000 can be readily reduced to a nodaldiagram with corresponding inputs, outputs, and interconnections asshown in FIG. 11. NOR logic gates 1001 through 1007 correspond to nodesA through G, respectively, and have been so designated. With the nodaldiagram of FIG. 11, an array of reflection subholograms may be made tooptically interconnect a corresponding array of optical logic NOR gatesto implement the clocked JK flip-flop circuit 1000.

Depicted in FIG. 12 is array 1200 of optically nonlinear optical devicessuch as SEEDs 1201 through 1209, also respectively designated A throughI, for implementing an optical version of clocked JK flip-flop logiccircuit 1000. In addition, this optical logic arrangement may also beconsidered an optical digital processor. Either an incoherent or acoherent light source (not shown) illuminates the front surface 1210 ofdevices 1201 through 1207 with bias beams 1251 through 1257,respectively. Devices 1208 and 1209 emit output light beams from frontsurface 1210 and are shielded from the light source, for example, by aplanar mirror. Bias beams 1251 through 1257 optically cause respectiveSEEDs 1201 through 1207 to function as optical logic NOR gates.

Reflection hologram 1220 splits and reflects the interconnecting outputlight beams from gate array 1200 to interconnect the optical NOR gatesto form a clocked JK flip-flop optical logic circuit similar to logiccircuit 1000. The reflection hologram comprises a plurality ofreflection subholograms 1221 through 1229 each uniquely associated witha correspondingly positioned SEED of logic gate array 1200. Opticaltransparent material 1240 maintains the front surface 1230 of reflectionhologram 1220 and the rear surface 1211 of optical NOR gate array 1200in a fixed parallel position with the front surface of each reflectionsubhologram directly facing the correspondingly positioned rear surfaceof the optical NOR gate.

As previously described, each reflection subhologram is exposed to atleast two generally opposing coherent light beams to form an opticalfringe pattern that is permanently stored in a well-known manner in aphotographic emulsion. After processing, the permanently stored fringepatterns become reflection subhologram 1221 through 1229.

For purposes of illustration, each reflection subhologram has an arrayof nine dots to indicate the relative center position of each opticalNOR gate from and to which optical signals are received and emitted.Again, the shaded box merely illustrates the relative position of theoptical NOR gate in array 1200 to which an output beam is reflected.

To verify the predetermined optical interconnections established by theoptical fringe pattern of each reflection subhologram, one need onlycompare the shaded boxes of each reflection subhologram for a particulargate with the outgoing lines from the corresponding node in FIG. 11. Thethree-by-three array of SEEDs 1201 through 1209 corresponds to thethree-by-three array of nodes A through H in FIG. 11, respectively. Theshaded box of reflection subhologram 1221 indicates that anyinterconnecting output beam from optical NOR gate 1201 (node A) will bereflected as an interconnecting control beam to optical NOR gate 1204.This is likewise indicated in the nodal diagram of FIG. 11. The opticalinterconnections can be similarly verified for reflection subholograms1222 through 1227.

Having verified the optical interconnections of reflection subholograms1221 through 1227, a functional comparison of the optical clocked JKflip-flop circuit of FIG. 12 can be readily made with the logic diagramof clocked JK flip-flop circuit 1000 depicted in FIG. 10. This is toverify that the two circuits function in an equivalent manner.

Briefly, the clocked JK flip-flop is functionally identical to thewell-known set-reset (SR) flip-flop except when the signals on the J andK terminals are asserted together. In addition, the future outputsignals of the JK flip-flop are a function of its present state. In thisexample, the JK flip-flop is designed to simply toggle or change stateswith the rising-edge of the clock signal should the signals on the J andK terminals be asserted at the same time. The SET and RESET inputs onthe JK flip-flop are provided to override the clocked inputs to thecircuit. However, to better understand the operation of this circuit,the logic levels on the (ET and RESET inputs will remain inactive.

Thus, applying a high logic level signal (H) to input terminals J andCLK and a low logic level signal (L) to input terminal K, the outputsignals on output terminals Q and Q assume high and low logic levels,respectively. This is similarly illustrated in FIG. 12 by applying highlogic level control beams 1261H and 1262H to respective optical NORgates 1201 and 1202 and a low logic level light beam 1276L to opticalNOR 1203. As a result, optical NOR gate 1207 emits a high logic levelinterconnecting output beam 1265H that reflection subhologram 1227splits and reflects as high logic level interconnecting feedback controlbeams 1266H and 1267H and high logic level interconnecting output beam1268H. Unbiased SEED 1208 passes high logic level output beam 1268H tothe Q output terminal.

As shown in FIG. 10, the state of interconnected NOR gates 1006 and 1007will determine what signals are fed back to NOR gates 1004 and 1005. Itis initially assumed that the output signal from NOR gate 1006 is at ahigh logic level (H) and fed back to NOR gates 1005 and 1007. And it isalso assumed that the output signal from NOR gate 1007 is at a low logiclevel (L) and fed back to NOR gates 1004 and 1006. Since the outputsignal from logic gate 1003 is at a high logic level (H), the outputsignal of NOR gate 1005 will be at a low logic level (L) regardless ofthe output signal fed back from NOR gate 1006 to an input of gate 1005.However, since two input control signals to NOR gate 1004 are at a lowlogic level (L), the output signal fed back from NOR gate 1007 willdetermine the logic level of the output signal from NOR gate 1004.First, since the logic level on the SET and RESET input terminals to NORgates 1006 and 1007 will force the gates to assume a predeterminedoutput level, these input terminals are assumed to be inactive or, inthis case, at a low logic level. Next, the output signal of gate 1004assumes a high logic level and is applied to the input of NOR gate 1006.This then causes a transition in the states of NOR gates 1006 and 1007such as to cause a high logic level signal from NOR gate 1006 to be fedback to NOR gate 1005. This finally results in the output signals of NORgates 1006 and 1007 assuming a low and a high logic level, respectively.

This condition can also be readily verified in FIG. 12. Thus, it can beseen that the optically interconnected clocked JK flip-flop circuit ofFIG. 12 functions equivalently to clocked JK flip-flop circuit 1000depicted in FIG. 10.

It is to be understood that the above-described optical combinationaland sequential logic circuits are merely illustrative embodiments of theprincipals of this invention and that any other optical logic circuitincluding a more complex optical digital processor may be devised bythose skilled in the art without departing from the spirit and scope ofthe invention. In particular, the arrays of reflection subholograms andoptically nonlinear optical devices may be utilized to perform memoryfunctions and parallel processing. Furthermore, the optical input andoutput signals of each array of logic gates may also be interconnectedor cascaded to form any size logic circuit desired.

What is claimed is:
 1. An optical logic arrangement comprising:a firstoptically nonlinear optical device responsive to a first control lightbeam for emitting a first output light beam; a second opticallynonlinear optical device responsive to a second control light beam foremitting a second output light beam, said first and second output beamsbeing a nonlinear gain function of said first and second control beams,respectively; and a reflection hologram responsive to said first outputlight beam for originating said second control light beam to said secondoptically nonlinear optical device.
 2. The arrangement of claim 1wherein said arrangement further comprises a third optically nonlinearoptical device responsive to a third control light beam for emitting athird output light beam, said third output beam being said nonlineargain function of said third control beam, and wherein said reflectionhologram is also responsive to said first output light beam fororiginating said third control light beam to said third opticallynonlinear optical device.
 3. The arrangement of claim 1 wherein each ofsaid first and second optically nonlinear optical devices is responsiveto a bias light beam for performing an optical logic function.
 4. Thearrangement of claim 1 wherein each of said devices comprises anonlinear interferometer.
 5. An optical logic arrangement comprising:afirst optical logic element responsive to a first control light beam foremitting a first output light beam; a second optical logic elementresponsive to a second control light beam for emitting a second outputlight beam, said first and second output beams being a nonlinear gainfunction of said first and second control beams, respectively; and areflection hologram responsive to said first output light beam fororiginating said second control light beam to said second optical logicelement.
 6. The arrangement of claim 5 wherein said arrangement furthercomprises first director means for directing said first control lightbeam to said first optical logic element.
 7. The arrangement of claim 6wherein said arrangement further comprises optically transparent spacermeans for maintaining the positions of said first and second logicelements relative to said reflection hologram.
 8. The arrangement ofclaim 5 wherein said arrangement further comprises director means fordirecting said second output light beam to an optical receiver.
 9. Thearrangement of claim 5 wherein each of said first and second opticallogic elements comprises a nonlinear interferometer.
 10. The arrangementof claim 5 wherein said arrangement further comprises a third opticallogic element responsive to a third control light beam for emitting athird output light beam, said third output beam being said nonlineargain function of said third control beam, and wherein said reflectionhologram is also responsive to said first output light beam fororiginating said third control light beam to said third optical logicelement.
 11. The arrangement of claim 5 wherein said reflection hologramcomprises means for reflecting a first predetermined amount of saidfirst output light beam as said second control light beam.
 12. Thearrangement of claim 11 wherein said arrangement further comprises athird optical logic element responsive to a third control light beam foremitting a third output light beam, said third output beam being saidnonlinear gain function of said third control beam, and wherein saidreflection hologram is also responsive to said first output light beamfor reflecting a second predetermined amount of first output light beamas said third control light beam to said third optical logic element.13. The arrangement of claim 12 wherein each of said first, second, andthird optical logic elements has a surface for receiving a control lightbeam thereon and emitting its output light beam therefrom.
 14. Anoptical sequential logic arrangement comprising:an optical logic elementresponsive to a first and a second control light beam for emitting anoutput light beam, said output beam being a nonlinear gain function ofat least one of said first and second control beams; and a reflectionhologram responsive to said output light beam from said optical logicelement for reflecting a predetermined amount of said output light beamback to said optical logic element as one of said first and secondcontrol light beams.
 15. The arrangement of claim 14 wherein saidoptical logic element has a surface for receiving said first and secondcontrol light beams thereon and emitting said output light beamtherefrom.
 16. An optical sequential logic arrangement comprising:afirst optical logic element responsive to a first and a second controllight beam for emitting a first output light beam, said first outputbeam being a nonlinear gain function of at least one of said first andsecond control beams; a second optical logic element responsive to athird control light beam for emitting a second output light beam, saidsecond output beam being said nonlinear gain function of said thirdcontrol beam; a first reflection hologram responsive to said firstoutput light beam for originating said third control light beam to saidsecond optical logic element; and a second reflection hologramresponsive to said second output light beam for originating one of saidfirst and second control light beams to said first optical logicelement.
 17. An optical logic arrangement comprising:a plurality ofoptical logic elements each responsive to a control light beam foremitting an output light beam, the output beam from at least one of saidelements being a nonlinear gain function of the control beam thereto;and a plurality of reflection holograms each responsive to the outputbeam from a specified one of said elements for originating a controllight beam to at least one other of said elements for establishing anoptical interconnection between the specified element and said at leastone other element.
 18. The arrangement of claim 17 wherein saidplurality of optical logic elements are positioned in a first plane andwherein said plurality of reflection holograms are positioned in asecond plane.
 19. The arrangement of claim 18 wherein said first andsecond planes are substantially parallel.
 20. The arrangement of claim18 wherein particular of said optical logic elements each has a surfaceon one side of said first plane for receiving an control light beamthereon and transmitting its output light beam therefrom.
 21. Thearrangement of claim 20 wherein said arrangement further comprises firstdirector means for directing a control light beam from an optical sourcereceived on the other side of said first plane to said optical logicelements.
 22. The arrangement of claim 21 wherein said arrangementfurther comprises second director means for directing an output lightbeam emitted from the other side of said first plane to an independentoptical receiver.
 23. The arrangement of claim 17 wherein each of saidoptical logic elements comprises a nonlinear interferometer.
 24. Anoptical digital processor comprising:optical combinational logic meansresponsive to first and second control light beams for emitting a firstoutput light beam, said first output beam being a nonlinear gainfunction of at least one of said first and second control beams; opticalsequential logic means responsive to a third control light beam foremitting a second output light beam, said second output beam being saidnonlinear gain function of said third control beam; a first reflectionhologram responsive to said first output light beam for originating saidthird control light beam to said optical sequential logic means; and asecond reflection hologram responsive to said second output light beamfor originating at least one of said first and second control lightbeams to said optical combinational logic means.
 25. The processor ofclaim 24 wherein said optical combinational logic means comprises firstand second optical logic elements each having a surface for receiving acontrol light beam thereon and emitting its output light beam therefrom.26. The processor of claim 25 wherein said optical sequential logicmeans comprises a third optical logic element and wherein each of saidfirst, second, and third optical logic elements comprises a nonlinearinterferometer.
 27. An optical logic arrangement comprising:a firstoptically nonlinear optical device responsive to a first control lightbeam for emitting a first output light beam; a second opticallynonlinear optical device responsive to a second control light beam foremitting a second output light beam, said first and second output beamsbeing a nonlinear gain function of said first and second control beams,respectively; a reflection hologram responsive to said first output beamfor originating said second control beam to said second device; andoptically transparent spacer material having oppositely facing first andsecond surfaces for maintaining the positions of said first and seconddevices relative to said reflection hologram, said reflection hologrambeing fixedly positioned on said first surface, said devices beingfixedly positioned on said second surface.
 28. An optical logicarrangement comprising:an array of optically nonlinear optical deviceshaving oppositely facing first and second surfaces, each of said deviceshaving a fixedly positioned first light-emitting and receiving area ofsaid first surface and a second light-emitting and receiving area ofsaid second surface opposite the first light-emitting and receivingarea, each of certain of said devices being responsive to a receipt of abias light beam at the second light-emitting and receiving area foremitting an output light beam from the first light-emitting andreceiving area thereof, each of said certain devices being responsive toa receipt of a control light beam at at least one of the first andsecond light-emitting and receiving areas for controlling the emittingof the output beam from the first light-emitting and receiving areathereof, the output beam from each of said certain devices being anonlinear gain function of the control light beam received thereat, eachof said certain devices being designated for optical interconnectionwith at least one other of said devices, each of others of said devicesbeing responsive to a receipt of a control light beam of one of thefirst and second light-emitting and receiving areas for emitting anoutput light beam from the other of the first and second light-emittingand receiving areas thereof, an array of reflection holograms includinga photosensitive material having a substantially flat surface facingsaid first surface of said devices, each of said holograms having anoptical fringe pattern formed within said photosensitive material andopposite an individual one of the first light-emitting and receivingareas of said first surface of said devices, each of said fringepatterns being effective in response to the output beam from one of saiddevices opposite the fringe pattern for originating an individualcontrol beam to each of other of said devices designated for opticalinterconnection with the one device, and an optically transparent spacermaterial for maintaining said first surface of said devices and saidflat surface of said photosensitive material in a fixed relativeparallel position, said spacer material having oppositedly facing firstand second substantially flat parallel surfaces, said flat surface ofsaid photosensitive material being fixedly positioned with said firstsurface of said spacer material, said first surface of said devicesbeing fixedly positioned with said second surface of said spacermaterial.